1. Field of the Invention
The present invention relates to optical fiber sensors and, more particularly, to distributed optical fiber chemical analysis.
2. Background Art
Simultaneous monitoring of multiple parameters is becoming a growing need in a variety of environments and applications. Distributed and quasi-distributed fiber-optic sensors capable of such functionality are one of the most promising approaches to address these issues and to provide new possibilities in intelligent monitoring systems.
Specialty optical fibers and micro-fabrication techniques make it possible to produce optical fibers with integrated transducer elements. This is especially interesting for chemical sensing with fiber-optic sensors. Chemical sensing represents certainly a potential for fiber-optic sensors, since it is essential in major application domains such as biomedical, food-processing, environment and industrial process monitoring.
In capillary column gas chromatography, a small quantity of a gas mixture is injected at the entrance of a capillary and carried through it by an inert gas flow, called the carrier gas. Along its path the mixture is separated into its components because each of them has a different rate of elution. These rates differ due to differences in the adsorption (gas-solid) or partition (gas-liquid) process that happens at the internal capillary wall. The type of process controlling the separation depends on the chemical composition of the internal capillary surface. For the adsorption process, layers of molecules settle directly on the solid surface as a condensate and for the partition process molecules are dissolved in a thin liquid polymer film. If the separation is effective, pure chemicals are detected one after the other at the exit of the capillary column.
In open tubular liquid chromatography (see Jinno, K. and H. Sawada (2000), “Recent trends in open-tubular capillary electrochromatography”, TrAC Trends in Analytical Chemistry 19(11): 664-675 and Swart, R., J. C. Kraak, et al. (1997), “Recent progress in open tubular liquid chromatography”, Trends in Analytical Chemistry 16(6): 332) the mobile phase carrying the sample to be analyzed in the column is liquid.
Other types of chromatographies using capillaries exist. Each capillary chromatography uses a reversible interaction of the chemical compounds dissolved in the mobile phase with the capillary surface or a stationary phase covering the capillary surface. The chemical interactions involved in the process comprise adsorption, dissolution, chemisorption, ionic exchange or any other interaction involving the reversible taking of matter by another matter. The term absorption is used herein to designate any of these or other chemical interactions. The term absorption is used to designate chemical absorption as opposed to light or power absorption.
Many chemical sensors based on micro-structured fibers measure the integral of the analyte optical properties (usually absorbance) along the fiber length. Therefore, for a given mixture, these sensors would register the same signal, whether or not the mixture has been separated. A chromatogram can be constructed by measuring the variation of this integral in time as the various separated components exit the capillary (see Xi, X. and E. S. Yeung (1990). “Axial-Beam On-Column Absorption Detection for Open Tubular Capillary Liquid Chromatography.” Anal. Chem. 62: 1580-1585). Another approach for real-time monitoring of a separation process is the use of a distributed sensor that allows monitoring of the separation process along the capillary.
One kind of distributed fiber optics sensor is the white-light polarimetric sensor (see Khomenko, A. et al. (1998). “Wavelength-scanning technique for distributed fiber-optic sensors”, Op. Lett., 18, pp. 2065-2067.). This kind of sensor makes use of a birefringent optical fiber placed between two linear polarizers. White light is injected at one end and the transmitted spectrum is recorded at the other end. Small local perturbations along the fiber create coupling between the two polarization modes carried by the birefringent fiber and each of these mode coupling events is contributing to the apparition of peaks in the Fourier spectrum of the transmitted signal.
Another option is a Mode-Filtered Light Detection system as provided in Zhou, L., K. Wang, et al. (2004); Synovec, R. E., C. A. Bruckner, et al. (1994); Foster, M. D. and R. E. Synovec (1996). A multimode optical fiber is inserted in a chromatography capillary and absorption of the analyte at the surface of the optical fiber results in higher-order modes leakage detected on the side of the optical fiber. In such a configuration, light detection could be very sensitive to light injection conditions and the system is likely to be very unstable. Such a system is also sensitive to fiber bending and it is thus not possible to wind the capillary column as is typically done in chromatography.